Suppose I intend to assassinate a man whose death, everyone agrees, would make the world a better place. Unfortunately, the only means I have to do so is a nuclear bomb. Knowing a little about nuclear bombs, I predict that its detonation will kill a million other people. Still, the villain needs to die. So I set off the bomb. Is it reasonable for me to claim afterwards that I didn't intend to kill the million other people, that they were regrettable "collateral damage" in my noble undertaking? Or should I say that, yes, I killed a million, but 20 million previously oppressed people will now live in liberty and comfort? Can I even say that I had a "moral right" to go nuclear?

To prod the reader into thinking through such arguments properly is the aim of philosopher Ted Honderich's deeply provocative and forceful new book. It expresses a bracingly airy contempt for our "conventions of idiocy", for our leaders, and for numerous other thinkers. Sometimes the tone is peremptory: "The general principle against violence can hardly be serious," Honderich writes, though it was apparently seriously meant by Jesus, whatever you think of him. The author can also be enjoyably catty - the Blair government is described as "inanely resolute" - but he always treats the reader with high intellectual civility.

What is to be our guide in deciding moral questions? International law, "just war theory", international relations, conservatism or liberalism, talk of democracy or freedom - our author considers them all, and chucks them all in the bin. What we need is his own Principle of Humanity. A "bad life" lacks one or more of the following goods: a decent length of life; the means to bodily quality of life; freedom and power. The Principle exhorts: "We must actually take rational steps to the end of getting and keeping people out of bad lives."

The Principle's first result is the most resistible. If you grant the Palestinians' moral right to national self-determination, and if terrorism is the only means for them to pursue that right, then you must grant that they have a moral right to their terrorism. Honderich emphasises that such a conclusion is "terrible", because terrorism is "prima facie wrong". But the conclusion depends also on a question of fact. Is terrorism really "the only possible means" available to the Palestinian people to alleviate their suffering?

The question is really three. First, how exactly is it to count as a "means"? In what reasonable way can a suicide bomber targeting teenagers at an Israeli nightclub hope that his or her actions will lead to nationhood? (The hope must be reasonable, on Honderich's own argument.) Second, has Palestinian terrorism in fact been such a means? Third, why are the efforts of Palestinian writers, lawyers, protesters or even politicians not allowed to count as means?

That terrorism is the only means is "evidently ... a factual proposition in need of support", Honderich notes. But the support is not here forthcoming. It is, Honderich says, "something about which myself I have no doubt". The reader may doubt that, in the absence of factual support in the text, the "terrible conclusion" amounts to much more than a defiant way of expressing one's sympathy for Palestinians. (The Principle also tells us, by the way, that the founding of the state of Israel and the "terrorism for it" were justified.)

Honderich goes on to describe 9/11 and 7/7 with appalling vividness. That they were wrong was "written on them", he says, a potent image that is almost out of a holy book. The Principle does not condone them; also, to pretend that they had nothing to do with what else has gone on in the world is fatuous. Yet his account of the context is askew: 9/11 is said "first of all" to have as its necessary cause the post-1967 actions of Israel in the occupied territories, even though the writings of founding militant Islamist Sayyid Qutb predate 1967, and even though Osama bin Laden was inspired first of all by Afghanistan in the 80s and Saudi Arabia and Bosnia in the 90s.

Of course, civilians are bombed by people other than suicide-bombers. Many more of them, for instance, are bombed by the US and UK governments, recently in Iraq. It is said that our killing of tens of thousands of civilians in Iraq is not comparable to killing by Palestinian or Saudi Arabian or British suicide bombers. The moral difference is that we somehow didn't intend to kill those civilians: their deaths were unfortunate accidents in our noble undertaking.

But the disavowal of intent is incoherent, and masterfully demolished by Honderich. If people in early 2003 were not stupid or amnesiac or drunk on fantasies of "smart weapons", they knew that invading a large country would result in a substantial civilian death toll. History, after all, is not ambiguous on this matter. If one thought war was a good idea anyway, then one cannot later claim not to have intended those deaths. To intend to do something that has the foreseeable consequence of many deaths is to intend that consequence. "It is nonsense to suppose," Honderich writes, "that something is to be judged right as a result of ignoring some of what you know or believe it will do."

Certain conclusions, not shyly expressed, follow. Iraq was a "terrorist war". The so-called "war on terror" has not addressed the "causes" of terrorism as it should have done. Our leaders "have been deficient in moral intelligence". Perhaps we should even refuse to pay our taxes. Maybe the reader will think Honderich is "an ideologue", but, he advises in conciliatory mood: "Such argy-bargy between us doesn't matter much. You were here for a kind of inquiry, here to look into things, here to hear what can be said for some propositions, here to follow some arguments."

Looking into things and following arguments might be our only hope. At times, indeed, Honderich expresses a wistfulness about impossible worlds: "The world isn't a university or a book or a half-decent discussion. If it could be, there are people who make sure it isn't." Well, this is a book, and readers who enjoy being goaded into thinking for themselves will enjoy a better than half-decent discussion with it.